1. Field of the Invention
The invention concerns a monitoring circuit for a plurality of simultaneously current-carrying valves.
2. Description of the Prior Art
In converters for high power, e.g., in a-c control elements, it is often necessary to connect several valves, in particular thyristors (SCR's), in parallel in one valve branch, if one valve alone is not capable of carrying or switching the desired current. All the valves of the valve branch should uniformly carry the same current.
In such a converter, there may occur a situation where at least one valve of the valve branch carries too large a current. This current can exceed the current-carrying capacity of the valve, so that it is in danger of being thermally destroyed. To prevent such failures, one can connect in series with each of the individual valves a fuse which responds in the event of an overload.
In such a converter, however, there may also occur an asymmetrical current distribution and where one valve carries too large a current, in comparison with the other valves, but is still within the permissible current-carrying capacity. Such a situation cannot be indicated nor prevented by fuses, although it of course would be desirable.
Finally, it may happen in such a converter that a valve takes no current or too little current in comparison with adjacent valves. The reason for this may be that the valve does not function because of a defect, in the case of a semiconductor valve, for instance, because of a contact failure (interruption) between its anode-cathode path. However, the reason may also be that a fuse in series with the valve in question has blown and the current path is interrupted. In a controlled valve, the reason may, finally, also be that the triggering of this valve was intermittent, for instance, due to a failure of the control unit or a fault in the control loop. In the last-mentioned situation the remaining parallel-connected valves of the valve branch must take over the current of the dropped-out valve. They are thereby stressed to the limit of their current carrying capacity or even beyond. Current interruption could heretofore be indicated only by a blown fuse. Also in such a situation, it is advisable to obtain a trouble alarm if a current interruption of the valve in question has occurred for any reason whatever, so that an alarm and/or protective measures can be released.
Accordingly, it is an object of the invention to provide a monitoring circuit for a number of simultaneously current carrying valves, which in the event of trouble of any kind in a valve, indicates this trouble or faulty loads in the other valves and delivers an alarm signal. It should make no difference whether the simultaneously current-carrying valves are controlled or uncontrolled electric valves such as, for instance, thyristors or diodes.